I agree with what you're saying and while I'm not advocating to remove 1-shot mechanics, I do have to point out that for people who can't grasp the 1-shot mech and keep getting hit by them just leads people to learn from external sources. Like me lol.
I mean, its either that or poke the next guy in my party and hope he knows how to dodge it. And its not like 24-man raids have all day to let you take your time to stand around and see the train coming at you so that you can be ready for next time. So for some, they're not gonna learn from the fight directly. Survivable but consequential dmg on the other hand is the kind of learning tool lets you do that imo. Especially if the punishment is aggravating that it can still lead to a wipe if you get into it again.
Again, the greater issue with 1-shot mechs is figuring out where it comes from. Dying instantly at something that barely registers your screen and doesn't give you enough time to pick up on clues leaves you puzzled and you can't really improve on information you don't have.